This invention relates to a packaging method using lead-free solder alloy metal the toxicity of which is minor, a soldering system, and a mounting structure in which the packaging method and soldering system are used. This lead-free solder alloy metal can be applied to connect an electronic device to a circuit board of an organic board and the like, and is used as a substitution of Sn-37Pb (Unit: Mass %) which is used for a soldering at about 220° C.
A conventional method to solder a device to a circuit board made of an organic material and the like of electric appliances comprises a reflow-soldering step in which hot air is blown against the circuit board, solder paste printed on electrodes is molten, and thus, a surface mount device is soldered, and a flow-soldering step in which a molten solder jet is caused to come into contact with a circuit board, and thus, some surface mount devices such as a insertion mount device, a chip device and the like are soldered.
And, this soldering method is called a packaging method.
Now, it has been requested to use lead-free solder alloy metal the toxicity of which is minor for both the solder paste used in the reflow-soldering step and the molten solder jet used in the flow-solder step.
As the prior arts related to this packaging method using lead-free solder, Japanese Patent Laid-open No. 10-166178 (Prior Art 1), No. 11-179586 (Prior Art 2), No. 11-221694 (Prior Art 3), No. 11-354919 (Prior Art 4), No. 2001-168519 (Prior Art 5), and No. 2001-36233 (Prior Art 6) and the like have been known.
In the prior art 1, for the lead-free solder, Sn—Ag—Bi system based solder or Sn—Ag—Bi—Cu based solder alloy metal is mentioned. In the prior art 2, it is mentioned that solder of Sn—Ag—Bi system which leads as a lead-free solder is connected to an electrode the surface of which is treated with a layer of Sn—Bi system. In the prior art 3, it is mentioned that electronic devices are reflow-soldered to both the 1st and 2nd surfaces of an organic board with lead-free solder which contains Sn as the main component, 0 to 65 mass % of Bi, 0.5 to 4.0 mass % of Ag, and total 0 to 3.0 mass % of Cu and/or In. In the prior art 4, it is mentioned that solder is cooled under an about 10 to 20° C./s cooling velocity in a method by which electronic devices are connected to a circuit board with Bi containing lead-free solder. In the prior art 5, it is mentioned that, in a method by which an electronic device is surface connected and mounted on surface A of a board by means of a reflow-soldering, and then a lead of the electronic device inserted from the surface A side is flow-soldered on surface B of the board, the solder used for the reflow-soldering in the surface A side is a lead-free solder of such a composition as Sn (1.5 to 3.5 wt %), Ag (0.2 to 0.8 wt %), Cu (0 to 4 wt %), and In (0 to 2 wt %), and the solder used for the flow-soldering in the surface B is a lead-free solder of such a composition as Sn (0 to 3.5 wt %), Ag (0.2 to 0.8 wt %), and Cu. In the prior art 6, it is mentioned that, when flow-soldering by using a lead-free solder of an eutectic crystal composition the melting point of which is higher than that of a conventionally used Sn-37Pb, it is so intended that a temperature difference between an organic board and an electronic device main body will not increase while cooling the board after flow-soldering by using a heat conducting material in a space between a device main body and a board.
In this packaging method, as mentioned in the prior Art 6, in case of a flow-soldering in which molten solder jet was caused to come into contact with the lower surface of a circuit board, a case in which soldering was made under a temperature higher than that of a conventional method by using a lead-free solder of an eutectic crystal composition with a higher melting point than 183° C. melting point of the Sn-37Pb (Unit: Mass %) or using a lead-free solder of a similar composition was considered.
However, the following points were not taken into considerations in anyone of the above-described prior arts 1 through 6.
In a packaging method, when preheating was made rapidly to shorten circuit board soldering time, or when a large volume of a component such as Pb which caused solder composition in a connected portion after the connection to considerably deviate from the eutectic crystal composition was contained in plating of an electrode, there was a possibility to cause the following three types of faulty soldering.
First of all, when flow-soldering a insertion mount device to a circuit board, a phenomenon called “lift off” in which a solder in the insertion mount device connected position is peeled off from an electrode on the board due to a heat capacity difference of each insertion mount device occurs.
Secondary, the solder in an already connected surface mount device is molten again, causing the surface mount device to be peeled off.
Tertiary, because of a heat capacity of the already connected surface mount device, a component with a low melting point on the side near the board causes segregation in the solder in a connected portion when cooling the board after a flow-soldering, causing the reliability to drop down remarkably not only at the time of a connection by means of soldering but also after the connection.
Further, when conducting a rapid preheating to shorten circuit board soldering time, the above describing three types of faulty soldering are likely to occur.
Furthermore, in a conventional flow-soldering step, the soldering condition must be changed after all the flowing boards are carried out, and in particular, when manufacturing small batches of a variety of products, the loss of much time occurs.